Railway block code

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The railway block code is a system of bell rings used in Great Britain to communicate between manually operated Signal Boxes (the system is not used in modern power signal boxes). Each signal box would have a bell circuit to the box on either side of it along the line. The equipment would consist of a plunger or key (rather like a Morse key) which if pressed, rings the bell in the neighbouring box. The neighbouring box would have a key for communicating back, therefore each box has a key which rings the bell in the other box. Where a signal box communicates with more than one other box (in either direction along the line, and possibly along a branch line too), each bell would have a different tone, so that the signalman could tell them apart by ear.

Although codes varied from region to region, the following shows a few selected UK Western region bell codes:

Code	Meaning
1 bell	Call attention
2 bells	Train entering section
2 - 3 bells (2 bells, pause, 3 bells)	Is line clear for light engine?
3 - 1	Is line clear for stopping passenger train?
4	Is line clear for express passenger train?
2 - 1	Train arrived
3 - 3	Blocking back
5 - 2	Release token
2 - 5	Token replaced
5 - 5 - 5	Opening signal box
6	Obstruction danger
16	Testing bells and instruments

These would be used as follows (assuming a simple exchange between two signal boxes on a plain section of line) between two signal boxes A and B. Assume a train is to pass from the block controlled by A to the block controlled by B. This must be agreed and permitted by the signalman in box B. Here is the sequence of bell codes and associated actions:

Box A sends 1 bell to box B. (Call attention.)
Box B replies with 1 bell to box A. (Acknowledgement, indicating readiness.)
Box A sends 4 bells to box B. (Is line clear for express passenger train?)
Box B sends 4 bells to box A. (Acknowledgement, giving permission for box A to signal the train into the block controlled by B.)
(Slightly later.) Box A sends 2 bells to box B. (Train entering section.) This is sent as the train passes box A and enters the block controlled by B.
Box B sends 2 - 1 bells to box A. (Only after confirming that the train has arrived complete and no portion of it remains within the block section between A and B. This could be done by the guard of the train notifying the signalman or by the signalman observing the tail-lamp.)